Metro cities could be on the hook for $1 billion or more in cleanup costs in coming years as they grapple with contaminated sludge in storm-water ponds that dot the metro area.

The main culprit: carcinogenic compounds known as PAHs, which until recently were found in common coal tar-based sealants used on driveways and parking lots. Unsafe levels of the compounds have been found in many of the metro area's 20,000 public collection ponds, which receive water from streets and parking lots after rainfall.

Testing of a handful of ponds has revealed the tip of the iceberg: Nine of 15 ponds sampled by the Minnesota Pollution Control Agency (MPCA) a few years ago had unsafe levels of the compounds.

When the city of Inver Grove Heights tested 10 ponds this year, it found two with unsafe levels. The cost to clean up just one of them, which had 7,000 cubic yards of tainted sediment, was estimated at $450,000.

"This is a huge cost to municipalities," said Al Innes, an MPCA pollution prevention specialist, which oversees storm-water runoff permits required in 233 larger cities statewide. The agency estimates that even if only 10 percent of the metro area's ponds have unsafe pollutant levels, it would cost more than $1 billion to clean them up.

Cities won't be able to put it off forever, either. The ponds periodically fill up with sediment and need to be regularly dredged to continue working properly.

Expensive disposal

Therein lies the problem: Usually, sediment from the ponds can be used as fill or spread on open ground. But sludge with unsafe levels of PAHs must be trucked to lined landfills, often tripling the usual cost of disposing of it, officials said.

PAH stands for polycyclic aromatic hydrocarbon. The compounds, which can cause cancer and kill pond aquatic life, are carried in runoff from driveways or parking lots sealed with coal-tar products.

About 25 cities -- nearly all in the metro area -- have tried to reduce future pond contamination by banning coal tar sealant in their cities.

As the rainwater collection ponds fill up, cities must sample the sediment before it can be disposed of under MPCA storm-water permits.

"A lot of cities are getting a nasty surprise" as they find PAH contamination and discover the cost, said Craig Johnson of the League of Minnesota Cities.

Cities from Inver Grove Heights to White Bear Lake to Waconia have sought state help to pay for proper disposal. The MPCA has approved $400,000 in disposal grants to those and three other cities in the past few years, said Don Berger, the agency's municipal division administrator. Other grants were awarded to Roseville, Circle Pines and Golden Valley.

When Inver Grove Heights discovered that it would cost an estimated $450,000 to dredge one large pond, then truck sediment to the nearby Pine Bend Landfill and spread it, the city had to postpone dredging plans, said city Engineer Tom Kaldunski.

"There was no way we could afford $450,000," he said. The pond was nearly full, but the city had to wait for a $76,000 MPCA grant this year, he said. Combined with $80,000 in city funds, the total will be enough to remove about a third of the contaminated sediment this winter, Kaldunski said.

"We can't do it all, but we can at least take a bite out of it," he said. He noted the city has nearly 600 runoff ponds, many of which are natural basins left by Ice Age glaciers. The city has created a utility fee to raise money for future dredging, he said.

Meanwhile, "a number of cities are putting dredging on hold," said engineer Randy Neprash of Stantec Co., a consultant for the Minnesota Cities Stormwater Coalition.

A cheaper way?

In the quest for a cheaper way to dispose of the contaminated sediment, many cities are watching an experiment in White Bear Lake. The city was the first to ban coal-tar sealants in 2009 after finding PAHs in a pond's sediment that would cost $250,000 to dispose of, said City Engineer Mark Burch.

Burch said the city obtained MPCA permission to use its $85,000 grant in 2011 to test an alternative disposal plan. The city kicked in $100,000 for the project, in which about 10,000 cubic yards of pond sludge was molded into a berm near the pond and covered with dirt and grass seed. Burch said pipes and testing equipment were installed to see if the PAHs stay in the berm or show up in water leaching toward aquifers.

Many cities that can't afford landfill disposal costs are hoping the state will offer more cleanup grants or that alternative disposal methods, such as White Bear's berm, are found, Neprash said. The Stormwater Coalition was formed in recent years as many cities became aware of the enormous disposal costs of contaminated pond sediment.

The Legislature has passed no more pond cleanup grants since 2009, and nothing is on the horizon in the coming session, said Rep. Bev Scalze, DFL-Little Canada, chief sponsor of the 2009 bill.